1. Field of the Invention
The present invention relates to a mechanically-coupled tuning fork-scanning probe vibrating system for use in a scanning probe microscope, and more particularly, to a mechanically-coupled tuning fork-scanning probe vibrating system of which quality factor is maximized or controlled in a wide range so that distances between a detection tip of a scanning probe and a surface of a specimen can be detected with high resolution or a detection time can be reduced.
2. Description of the Related Art
Near-field scanning optical microscopes (NSOM) are a scanning probe microscope type in which a very small hole having a thickness equal to or less than 100 nm is perforated in a long probe having a sharp tip and the height of a surface of a specimen to be observed and optical characteristics of the specimen are detected while the probe is relatively moved with respect to the surface of the specimen forwards and backwards and right and left. General optical microscopes are based on detection of a far field and thus, diffraction of light occurs so that the optical microscopes have a resolution limit of about 200 nm. On the other hand, in NSOMs, a near field is detected by using a scanning probe having a numerical aperture (NA) that is equal to or less than 100 nm. Thus, the resolution limit of general optical microscopes can be overcome, and information about a three dimensional shape of the surface of the specimen and optical information thereof can be simultaneously detected.
FIGS. 1A and 1B schematically illustrate a conventional mechanically-coupled tuning fork-scanning probe vibrating system. Referring to FIGS. 1A and 1B, the conventional mechanically-coupled tuning fork-scanning probe vibrating system includes a tuning fork 10 having a detection circuit (not shown) installed therein and detecting variation of amplitudes and phases of the system, and a scanning probe 20 attached to the tuning fork 10 via an adhesive 30 and including a detection tip 22 having an end of a cross-sectional size of several microns or equal to or less than several microns. The system illustrated in FIGS. 1A and 1B detects variation of shear forces applied between the scanning probe 20 and the surface of a specimen and adjusts distances between the scanning probe 20 and the surface of the specimen up to a nanometer scale. The shear force is the Van der Waal's force that is generated between the detection tip 22 and the surface of the specimen when the scanning probe 20 having a length equal to or less than 20 nm is proximate to the surface of the specimen.
In connection with adjusting distances between the scanning probe 20 and the surface of the specimen in the system of FIGS. 1A and 1B, first, an AC voltage having a predetermined frequency is applied to the tuning fork 10 to which the scanning probe 20 is attached, so that the tuning fork 10 and the scanning probe 20 vibrate. Then, when the scanning probe 20 is proximate to the surface of the specimen and vibration of the scanning probe 20 is reduced by the shear force applied between the scanning probe 20 and the surface of the specimen, reduced vibration of the tuning fork 10 is detected by the detection circuit as variation of amplitudes and phases of the tuning fork 10 so that the distances between the scanning probe 20 and the surface of the specimen can be adjusted to a nanometer scale.
When the frequency of the AC voltage applied to the tuning fork 10 varies and an output voltage of the detection circuit installed in the tuning fork 10 is detected, the highest output voltage is recorded at a resonant frequency of the conventional mechanically-coupled tuning fork 10-scanning probe 20 vibrating system, and as the frequency of the AC voltage applied to the tuning fork 10 is farther away from the resonant frequency of the conventional mechanically-coupled tuning fork 10-scanning probe 20 vibrating system, the output voltage of the detection circuit installed in the tuning fork 10 is reduced. The output voltage according to the frequency of the AC voltage applied to the tuning fork 10 is referred to as a frequency response curve of the conventional mechanically-coupled tuning fork 10-scanning probe 20 vibrating system. In this case, a value that is obtained by dividing the resonant frequency of the conventional mechanically-coupled tuning fork 10-scanning probe 20 vibrating system by a half width of the frequency response curve is referred to as a quality factor. Accuracy of adjusting the distances between the detection tip 22 of the scanning probe 20 and the surface of the specimen is determined by the quality factor of the conventional mechanically-coupled tuning fork 10-scanning probe 20 vibrating system. That is, when the detection tip 22 of the scanning probe 20 having a length equal to or less than 20 nm is proximate to the surface of the specimen, the physical characteristics of the conventional mechanically-coupled tuning fork 10-scanning probe 20 vibrating system vary due to the effect of the shear force, and the resonant frequency of the conventional mechanically-coupled tuning fork 10-scanning probe 20 vibrating system varies, and the frequency response curve is moved left or right. In this case, the frequency of the AC voltage applied to the tuning fork 10 is the resonant frequency of the conventional mechanically-coupled tuning fork 10-scanning probe 20 vibrating system when the shear force is not detected, i.e., before the physical characteristics of the conventional mechanically-coupled tuning fork 10-scanning probe 20 vibrating system vary due to the shear force. Thus, as the frequency response curve is moved left or right, the output voltage of the tuning fork 10 is reduced. In connection with accuracy of adjusting the distances between the detection tip 22 of the scanning probe 20 and the surface of the specimen, when the quality factor of the conventional mechanically-coupled tuning fork 10-scanning probe 20 vibrating system is large, variation of the output voltage of the tuning fork 10 due to the shear force becomes larger. Thus, variation of the distances can be more accurately detected.
In other words, as the quality factor of the conventional mechanically-coupled tuning fork 10-scanning probe 20 vibrating system increases, the shear force may be more sensitively detected in the scanning probe microscope, which means that the distances between the detection tip 22 of the scanning probe 20 and the surface of the specimen can be more accurately adjusted.
However, when the scanning probe 20 is attached to the tuning fork 10 like in the conventional mechanically-coupled tuning fork 10-scanning probe 20 vibrating system illustrated in FIGS. 1A and 1B, the quality factor of the system is decreased by about 1/20 or less as compared to a natural quality factor of the tuning fork 10, because unbalance occurs in mass of two prongs of the tuning fork 10, a resistive force to be applied to the tuning fork 10 increases and a natural frequency of the conventional mechanically-coupled tuning fork 10-scanning probe 20 vibrating system is different from that of the tuning fork 10 and a loss of energy required for vibration of the conventional mechanically-coupled tuning fork 10-scanning probe 20 vibrating system occurs. In particular, in the NSOM, the scanning probe 20 having a large length is used and thus, a reduction in quality factor is severe. Thus, the distances between the detection tip 22 of the scanning probe 20 and the surface of the specimen may not be accurately adjusted.
In particular, when a biological specimen included in liquid is detected using the vibrating scanning probe 20, due to viscosity of the liquid, a large resistive force is applied to the scanning probe 20 in the liquid. Thus, the quality factor of the conventional mechanically-coupled tuning fork 10-scanning probe 20 vibrating system is greatly decreased, or the tuning fork 10 does not vibrate. In the conventional mechanically-coupled tuning fork 10-scanning probe 20 vibrating system illustrated in FIGS. 1A and 1B, it is difficult to detect the shape of the specimen in the liquid with high resolution.
FIGS. 19 and 20 illustrate a mechanically-coupled tuning fork-scanning probe vibrating system according to comparative examples.